Combustion exhaust gas including sulfur oxides, such as SO2 and SO3, is emitted from a combustion furnace in which fuels containing at least 0.5 wt % sulfur content, such as heavy fuels or coal fuels, are combusted.
SO3 is produced as a result of partial oxidation of SO2 at high temperatures. Therefore, the presence of SO3 is about a few percent relative to SO2. Nevertheless, the concentration of exhaust SO3 should be controlled below a few ppm because SO3 not only causes air heaters to clog or erode or chimney flues to erode but also causes blueish smoke to occur when cooled and discharged from a chimney.
Whereas well-known methods for removing SO3 include the ammonia injection method in which ammonia gas is blown into combustion exhaust gas, the present inventors have proposed a method for spraying desulfurizing effluent including a dissolved salt, such as Na2SO4, into combustion exhaust gas (hereinafter, referred to as the “dissolved-salt spray method”) as a technique for removing SO3 easily and at a low cost (refer to Patent Document 1).
Patent Document 1:
Japanese Unexamined Patent Application, Publication No. 2006-326575